Conventional heating, typically comprising convectional or convectional/radiative gas or electric resistance heating, is commonly used to manufacture ceramic materials. However, the slow heating rate and poor temperature control associated with conventional heating methods results in a high energy consumption and inconsistent product quality.
Industrial heating by microwave radiation has been successfully used to accelerate the slip casting and drying of traditional ceramics. In comparison with conventional heating, microwave heating can provide a higher heating rate, where there is sufficient microwave absorption, with better temperature control, and thus results in a lower energy consumption and potentially a better quality product.
Devices which utilize a combination of conventional and microwave heating are known for use at relatively low temperatures. A common example is the combination microwave/convection oven for cooking food. Convection heating is employed for uniformity of cooking and for purposes of enhancing flavour.
Some industrial processes use a combination of convection and microwave heating, also at relatively low temperatures. For example, U.S. Pat. No. 4,375,441 discloses a combination of microwave and conventional heating to obtain uniform sintering of large, complexly configured or non-homogeneous polymeric articles in order to avoid overfusing of the interior due to uneven heating which may occur if only microwave heating is used. Conventional heating may thus be applied either prior to or subsequent to microwave heating.
However, the dielectric loss factor (a measure of microwave absorption) of most ceramic materials is heavily dependent on temperature. Most ceramic materials do not permit significant microwave coupling (microwave absorption) at low temperatures. Since the benefits of microwave heating can only be fully realized when the ceramic material being processed permits significant microwave coupling, it has been a common practice in microwave processing of ceramics to use additives or coupling agents to enhance microwave coupling.
It has been found that microwave coupling increases dramatically when the temperature of the ceramic is elevated above a threshold temperature, which varies according to the material being processed, at which the dielectric loss factor increases significantly.
By using a combination of conventional and microwave heating, the present invention more efficiently processes ceramic materials. According to the invention, conventional heating is applied to elevate the temperature of ceramic materials to the threshold at which there is a significant increase in microwave coupling, at which point microwave heating is applied. The invention thus reduces both processing time and energy consumption as compared to conventional heating or microwave heating processes.
The process of convectional or convectional/radiative heating followed by microwave heating also provides the advantage that the conventional heating phase elevates the ambient temperature of the workspace, thereby reducing the temperature gradient between the interior and the surface of the ceramic materials during the microwave heating phase. A better quality product should result.
Those skilled in the art will recognize that within the temperature ranges referred to in this specification the radiative component of energy produced by a conventional heating source, such as an electric resistance heating element, is insignificant. However, in certain materials the dielectric loss factor may not increase significantly until much higher temperatures are obtained, in which case the radiative (infrared) component may become significant. For this reason conventional heating as used herein includes convectional and radiative heating even though in the examples given below convectional heating is the only significant form of heat transfer in the conventional heating phase.